1. Field of the Invention
The present invention relates generally to heat exchangers and, more particularly, to a steam coil apparatus having opposed inlet headers which feed steam into alternating rows of tubes in order to provide for more even distribution of the steam through the apparatus.
2. Discussion of the Prior Art
It is known to provide a steam heater including a plurality of generally horizontally extending outer tubes extending across a region past which air flows during heating of the air. In such devices steam and the resulting hot condensate water is delivered to the outer tubes via inner tubes received therein. The inner tubes receive steam from a laterally disposed inlet header, and are provided with spaced-apart openings which discharge steam into the concentric outer tubes. An example of such a device is illustrated in U.S. Pat. No. 2,991,978 to Jones, issued Jul. 11, 1961.
This general type of device remains in use today in ventilators, heaters and air conditioning systems, for transferring heat between the steam supplied to the apparatus and air flowing past the device. During normal operation, at steam pressures of 2-5 psig or greater, steam from the inlet header fills the inner tubes and is forced somewhat evenly under the pressure into the concentric outer tubes and delivered from the device. This even distribution of the steam through and along the inner tubes provides an even distribution of heat across the fins connected to the outer tubes so that the temperature of air passing the device is raised evenly, with few isolated, localized hot spots or cold spots forming.
A drawback to the conventional construction arises when the pressure of the steam fed to the apparatus drops below the design full load operating pressure, and is magnified at very low pressures of below 2 psig. It is often necessary to operate a steam coil apparatus at such low pressures in order to obtain the desired heating capacity of the heat exchanger. However, when such low pressures are employed in the conventional construction, hot spots and cold spots develop in the streams of air passing across the device, which can cause the system which the apparatus is installed within to malfunction or freeze under certain circumstances.
For example, it is known to employ a steam coil in a ventilation system for hospitals, laboratories or the like, wherein the steam coil is used to warm incoming, outdoor air to a desired, controlled temperature (e.g. 45.degree. F.) as the preheat coil in the air conditioning system which also utilizes chilled water coils in the same system. Typically, the chilled water coil is located from 1-6 feet downstream of the heating coil such that any stratification of temperature across the air stream remains substantially unchanged between the steam coil and the chilled water coil.
Where a safety assembly is included in the air conditioning system for shutting down the system when the temperature of the air entering the chilled water coil is below a predetermined temperature (such as 38.degree. F.) the possibility arises that cool air from within a localized cold spot will trip a sensor of the safety assembly causing the system to shut down even though the average temperature of the air reaching the chilled water coil is still at the desired operating temperature. Alternatively, without safety shutdown, the localized cold spot could result in freezing in the chilled water coil and resulting damage and repairs.
These localized hot spots and cold spots are created by uneven distribution of steam within the steam coil. At low pressures, steam fed to the inlet tubes of the conventional construction travels to the distal ends of the tubes before passing through the holes to the concentric outer tubes. Or, in the case of a steam coil with inner tubes supplied from both ends; steam fed to the inlet tubes travels to the midpoint of the tube length before passing through the holes to the concentric outer tube. This uneven distribution which occurs within each inner tube is due to the inertia of the fluid and causes the temperature at the distal end of each tube or the midpoint, in the case of the coil supplied from both ends, to rise above the temperature at the proximal end thereof. Because steam is fed to all of the inner tubes in the same direction, the temperature along the lateral side of the steam coil aligned with the distal ends of the inlet pipes increases or the midpoint, in the case of the coil supplied from both ends, while the temperature along the opposing lateral side of the steam coil drops.
When the temperature of the incoming, ambient air drops below freezing, and the steam coil is operated at lower than design steam supply pressures, the localization of heat at the distal ends of the inlet tubes heats the air to a temperature much greater than that desired, while air adjacent the proximal ends of the inlet tubes may allow water within the steam coil or the downstream chilled water coil to freeze. This extreme variation within the steam coil can cause failure of the device, or of the downstream system components (e.g. chilled water coil), or safety shutdown of the system.